Field of the Invention
The present invention relates to a die cushion force control method and a die cushion device, and more specifically relates to a technique that generates predetermined die cushion force without response delay of the die cushion force.
Description of the Related Art
Conventionally, there has been proposed a servo die cushion device that drives a servo motor according to a die cushion force command previously set to generate die cushion force on a cushion pad (Japanese Patent Application Laid-Open Nos. 2006-315074, 2012-240110).
As illustrated in FIG. 7, at the time of starting die cushion force action (a press slide collides with the cushion pad with an upper die, a material, a blank holder, and a cushion pin interposed therebetween), the servo die cushion device requires a die cushion force response time a of about 0.03 second to 0.1 second (delay time required to enhance force) in order to enhance force (increase force) from a state (position control state) that load A balancing with gravity acting on a die cushion related movable mass including the material, the blank holder, the cushion pin, and the cushion pad, to predetermined die cushion force B (B: it is from several times to several tens times larger than the load A).
Further, there is a problem that impact (force) occurs due to the collision of the press slide at the time of starting the die cushion force action and thereby the die cushion force becomes larger (overshoot) or smaller (undershoot) than the die cushion force command (set force).
In order to decrease the impact (force) due to the collision of the press slide, preliminary acceleration of the cushion pad has been generally performed (Japanese Patent Application Laid-Open No. 2007-301599 and the like).
Herein, as illustrated in FIG. 8, when a preliminary acceleration time is represented by b, a die cushion force action completion (press slide) position is in a position descended from a die cushion waiting (press slide) position by moving velocity of the slide×time (a+b), depending on the total time (a+b) between the preliminary acceleration time b and the die cushion force response time a.
Due to a die cushion force response distance X, or a sum of a die cushion preliminary acceleration distance Y and the die cushion force response distance X, force to press an edge of the blank (material) is insufficient when the drawing is started (beginning of the drawing) and thus a wrinkle (drawing wrinkle) is generated in a radial direction from the edge of the material, and the drawing is continued (drawing proceeds) in a state that the wrinkle is generated, thereby, at the time of drawing, not only impairing a product shape but also causing damage (galling) on forming surfaces of dies (upper and lower) because of unevenness of the wrinkle.
At the time of stretch forming, force to press the edge of the blank (material) is also insufficient when the stretch forming is started (beginning of the stretch forming), and thus a “stretch” is lacked in a stretch part of the product. This causes a defective product, for example an outer plate of a hood of a car formed by the stretch forming, which is bowed inward by being pushed lightly by a finger. Thus, in the stretch forming, the press force at the beginning of the start of the stretch forming is especially important. It is generally thought that the servo die cushion accompanying the die cushion force response distance X and the die cushion preliminary acceleration distance Y is difficult to perform the stretch forming by the use of at least a conventional die structure (a height of an upper surface of the blank holder corresponds with a height of an upper surface of a convex die at the time of starting the stretch forming).
That is, in this interval (die cushion preliminary acceleration distance Y+die cushion force response distance X), the die cushion force action is insufficient in the drawing, and it is impossible to perform the stretch forming.
In the case where the preliminary acceleration is not performed as illustrated in FIG. 7, the die cushion force action can be obtained earlier as compared with the case where the preliminary acceleration is performed as illustrated in FIG. 8. However, the die cushion force response distance X that depends on the response time a generates. Then, insufficient responsiveness for at least the stretch forming is achieved. Further, the impact force such as overshoot or undershoot of the die cushion force due to the collision generates in this case.
Meanwhile, in the case of a pneumatic (ordinary pressure) die cushion device, a cushion pad waits in a state that it has already applied force to an upper side machine limit (stopper) and thus the die cushion force action has been completed in the die cushion force action start position. The pneumatic die cushion device has no response delay accompanied by the die cushion force action, unlike the servo die cushion device, and thus can perform the drawing and the stretch forming. However, it goes without saying that there is a problem from another point of view (for example, pressure (force) is enhanced proportionally to a slide stroke), and thus the servo die cushion device has been produced.